In orthopaedic arthroplasty procedures, it is generally desirable to be able to determine the outcome of the procedure. For example, the intention of the procedure may be to recreate the state of the joint immediately prior to the procedure, the intention may be to put the joint into an original ideal or preferred state, or the intention may be to put the joint into a state which is preferred for the particular patient's anatomy or for other reasons. Whatever the specific intention of any particular procedure it is generally of used to be able accurately to determine the effect of the procedure on the joint.
For example, when carrying out a hip procedure, it is often desirable to be able to determine any changes in the leg length and/or offset caused by the procedure.
U.S. Pat. No. 6,711,431 describes a computer assisted surgical procedure which can be used to determine any change in leg length and offset. The system includes a locating system and a computer and a software module, executable on the computer. A pelvic tracking marker is fixed to the pelvic bone and a femoral tracking marker is securely attached to the femur of the patient. The position of the femur is determined before the operation with the femur in a specific reference position. The position of the femur in the same reference position is determined after the operation and the system detects changes in leg length and femoral offset. However, the accuracy of the determination depends on whether the femur has been returned to the same reference position and so in practice is subject to errors.
It is therefore desirable to provide a method having improved accuracy and reliability of determination of any change in a joint after a surgical procedure on the joint.